The present invention relates generally to radio or wireless communications and, more particularly, relates to a transmitter having a modified translation loop architecture.
Wireless or radio frequency (RF) communication systems are an integral component of the ongoing technology revolution and are evolving at an exponential rate. Many wireless communication systems are configured as xe2x80x9ccellularxe2x80x9d systems, in which the geographic area to be covered by the cellular system is divided into a plurality of xe2x80x9ccellsxe2x80x9d. Mobile communication devices or stations (e.g., wireless telephones, pagers, personal communications devices and the like) in the coverage area of a cell communicate with a fixed base station or transmitter within the cell. Low power base stations are utilized, so that frequencies used in one cell can be re-used in cells that are sufficiently distant to avoid interference. Hence, a cellular telephone user, whether mired in traffic gridlock or attending a meeting, can transmit and receive phone calls so long as the user is within a cell served by a base station.
The communication format used in most wireless communications systems is a high-frequency carrier waveform modulated by low frequency or xe2x80x9cbasebandxe2x80x9d audio or data signals. Mobile stations (wireless handsets, for example) within a wireless communication system typically have a transmitter that xe2x80x9cmodulatesxe2x80x9d baseband signals (e.g., speech detected by the handset microphone) onto the carrier waveform. Amplitude modulation (AM) and frequency modulation (FM) techniques, for example, are well known to those of ordinary skill in the art. Mobile stations also typically have a receiver that xe2x80x9cdemodulatesxe2x80x9d the carrier waveform to extract the baseband signal.
The carrier waveform that is modulated is usually a high frequency, periodic waveform generated by an oscillator. The frequency of the oscillator should be adjustable since the transmitter is often required to transmit on many different frequency channels within a transmission band. In a GSM wireless network, for example, the transmission band is 880-915 MHz and is divided into 200 kHz channels. Thus, the oscillator frequency must be varied in precise steps of 200 kHz. Voltage-controlled oscillators (VCOs) are well suited for such applications since their output frequency is easily adjusted by manipulating a control voltage.
Many transmitters use phase-locked loops (PLLs) to generate the desired range of frequencies. The design and operation of PLLs is well known to those of ordinary skill in the art. A basic PLL 10 is illustrated in FIG. 1. It includes a VCO 16 that outputs a signal having a frequency fTx within a defined transmission frequency band. PLL 10 also uses a reference or clock signal having a frequency fREF equal to the required step size or frequency resolution (e.g., the channel bandwidth) of the PLL. Each frequency channel (e.g., 900 MHz, 900.2 MHz, 900.4 MHz) is an exact integer multiple of the reference frequency (e.g., 0.2 MHz). VCO 16 locks to the reference signal and tracks any modulation contained in the reference signal (to the extent that it is passed through the loop filter).
Frequency divider 18 divides the frequency of the VCO output signal by an integer N to yield a signal having the same frequency as the reference frequency,       f    REF    =                    f        Tx            N        .  
The divided frequency and reference frequency signals are input to phase detector 12, which compares the phases of the two signals and outputs a control voltage to control the frequency of the VCO. The output from phase detector 12 is usually passed through a charge pump/loop filter (14) before being supplied to VCO 16. Hence, the output frequency of VCO 16 can be programmed in discrete steps by changing the value of divider N. Passing the reference frequency through a reference divider (not shown), thereby making the step size programmable, can provide additional flexibility.
Some wireless transmitters use a translation loop architecture that is very similar in concept and operation to a PLL architecture. An exemplary translation loop transmitter 200 is illustrated in FIG. 6. Transmitter 200 will be discussed briefly for now and in more detail in the description to follow. In a translation loop transmitter, modulation is typically performed by a quadrature mixer (202) that modulates baseband audio and/or data signals onto a carrier wave at an intermediate frequency (IF) generated by a local oscillator (LO1). The IF frequency is also used as the reference input to a phase detector (204). The frequency of the modulated VCO (210) output signal is down-converted to the IF frequency for phase comparison by mixing the VCO signal (mixer 212) with a signal generated by a second local oscillator (LO2).
Translation loop transmitters have several drawbacks. Frequency mixers generate various cross products of the local oscillator signals and their harmonics. Spurious mixing products can also be created through leakage of local oscillator signals. In FIG. 6, for example, the signal from oscillator LO1 may leak to oscillator LO2, and vice-versa, to generate mixing products. Though filters are employed to remove these mixing products, low frequency products (xe2x80x9czero crossingxe2x80x9d spurs) may not be attenuated by the filters and may cause corruption and spurious modulation of the VCO transmit signal. Additionally, translation loop transmitters generally require use of a quadrature mixer or modulator, which increases the required circuitry and decreases the cost efficiency of the transmitter.
The present invention provides a modified translation loop transmitter that improves on conventional translation loop transmitters. The transmitter has a VCO and one modulated signal source. Elimination of the multiple local oscillators used by prior art transmitters is cost-effective and reduces the occurrence of low frequency mixing product spurs. Moreover, the VCO transmit frequency is a multiple of the modulated signal source frequency, and is easily programmable to a particular frequency channel through the setting of the modulated signal (LO) frequency, or by changing the divider and multiplier factors, which are integers. Using a modulated signal source also eliminates the need for a quadrature mixer, which further reduces the cost and circuitry required.
In one embodiment of the present invention, a transmitter is provided. The transmitter comprises a voltage-controlled oscillator (VCO) that generates a transmit signal having a frequency fTx, and a modulated signal source that generates an LO signal having a frequency fLO modulated with audio or data information. The transmitter further comprises a mixer that mixes the transmit signal with the LO signal to produce an IF signal having a frequency fIF. A first divider divides the LO signal frequency fLO by an integer M to generate a first comparison signal having a frequency fCF, and a second divider divides the IF signal frequency fIF by an integer N to generate a second comparison signal also having a frequency fCF. The first and second comparison signals are supplied to a phase detector that compares the phases of the signals and outputs a control voltage to the VCO proportional to any phase differences between the signals.
In one implementation, the modulated signal source is a direct digital synthesizer (DDS) or a sigma-delta synthesizer. In a further implementation, a multiplier is provided that multiplies the LO signal by an integer K before the LO signal is supplied to the mixer. In this implementation, when fLO greater than fTx, the relationship between the transmit and LO signal frequency is expressed as             f      Tx        =                  f        LO            ⁢              xe2x80x83            ⁢                                    K            ⁢                          xe2x80x83                        ⁢            M                    -          N                M              ,
and when fLO less than fTx, the relationship between the transmit and LO signal frequency is expressed as       f    Tx    =            f      LO        ⁢                            KM          +          N                M            ·      
In another embodiment of the present invention, a wireless communication device is provided. The wireless communication device comprises a processor that directs the overall operation of the communication device. It further comprises a microphone for capturing audio acoustic signals and converting the acoustic signals into electric signals, a speaker for converting electric signals into audible acoustic signals, and an antenna for wireless transmission and reception of acoustic or data signals.
A receiver is provided for receiving audio or data information from the antenna and a transmitter is provided for transmitting audio or data information over the antenna. The transmitter has a modified translation loop architecture and comprises a voltage-controlled oscillator (VCO) that generates a transmit signal having a frequency fTx, and a modulated signal source that generates an LO signal having a frequency fLO that is modulated with audio or data information. The transmitter further comprises a mixer that mixes the transmit signal with the LO signal to produce an IF signal having a frequency fIF. A first divider divides the LO signal frequency fLO by an integer M to generate a first comparison signal having a frequency fCF, and a second divider divides the IF signal frequency fIF by an integer N to generate a second comparison signal also having a frequency fCF. The first and second comparison signals are supplied to a phase detector that compares the phases of the signals and outputs a control voltage to the VCO proportional to any phase differences between the signals.
The present invention also provides a method for transmitting a modulated transmit signal. The method comprises the following steps:
(a) generating a modulated LO signal having a frequency fLO;
(b) generating a modulated transmit signal having a frequency fTx=fLOxc2x7y;
(c) dividing the frequency fLO of the LO signal by an integer M to generate a first comparison signal;
(d) mixing the LO signal with the transmit signal to generate an IF signal;
(e) dividing the frequency of the IF signal by an integer N to generate a second comparison signal;
(f) comparing the phases of the first and second comparison signals; and
(g) adjusting the frequency fTx of the transmit signal if necessary to correct any phase differences between the first and second comparison signals.
In another embodiment of the present invention, a method is provided for selecting a transmit frequency channel fTx. The method comprises the following steps:
(a) selecting an initial transmit frequency channel fTx;
(b) providing a multiplication factory;
(c) selecting an LO signal having a frequency fLO such that fTx=fLOxc2x7y;
(d) generating a transmit signal on the frequency channel fTx; and
(e) repeating step (c) to change the frequency channel, and repeating step (d) to keep the same frequency channel.
Objects and advantages of the present invention include any of the foregoing, singly or in combination. Further objects and advantages will be apparent to those of ordinary skill in the art, or will be set forth in the following disclosure.